Many steel rail-wheel transportation systems including freight, passenger and mass transmit systems suffer from the emission of high noise levels which cause a nuisance to persons dwelling close to such systems. The origin of such noise emission can be directly attributed to the fact that under certain conditions arising in use, the wheels of such systems do not always roll over the rails but sometimes slide relative to them. This arises, for example, when negotiating curves where true rolling contact could only be maintained on both rails if inner and outer wheels could rotate at different peripheral speeds. As this is not possible on fixed axle traction or rolling stock, the wheels undergo a combined rolling and sliding movement relative to the associated rails. Sliding movement also arises on inclines where a point may be reached at which traction is lost causing driving wheels to slip. When negotiating curves and inclines therefore the wheels, particularly of the traction vehicle, undergo alternate rolling and sliding motions, this phenomenum being referred to as "slip-stick". The magnitude and speed of the sliding movement is dependent on the difference, expressed as a percentage, between the rail and wheel velocities at the point of contact. This percentage difference is termed creepage.
Apart from generation of noise, creepage also produces wear both of the wheels and of the rails. This wear is accentuated by persistent to and fro movement which results from the presence of clearances necessary to enable the train to move over the track. These effects together produce undulatory wave patterns on the rail surfaces which are generally referred to as corrugations. When this occurs the noise levels are increased beyond those for a smooth rail/wheel interface and ultimately the problem can only be cured by grinding or machining the rail and wheel surfaces. This is both time consuming and expensive and requires to be carried out periodically as corrugations again build up over time.
At creepage levels larger than about 1% appreciable frictional forces are generated due to sliding. One means of improving the situation would be to reduce the extent to which creepage occurred by increasing friction between rail and wheel before slipping takes place. This is particularly applicable in relation to slippage arising through gradients in the track, but creepage arising from the behaviour of solid axle wheel sets on bends cannot be eliminated in this way and can therefore only be reduced by lubrication control. However conventional lubrication which serves simply to reduce frictional resistance between rail and wheel can interfere with traction, deceleration and braking. There is accordingly a conflicting requirement on the one hand to increase friction to reduce slipping or creepage and on the other hand to reduce wear by lubrication control.
In International Patent Application No. PCT/GB90/00878 published under No. WO 90/15123, there is disclosed a lubricant composition having a high and positive coefficient of friction, that is having the property whereby, contrary to conventional lubricants, the coefficient of friction between the lubricated surfaces increases with speed of relative sliding movement. The composition disclosed in the aforesaid International Application is designed primarily to reduce wear arising from the slip-stick effect although contrary to conventional lubricants it produces an increase in frictional force as slippage occurs.
While this composition is extremely effective in responding to slippage as it occurs, it places less emphasis on the prevention of slippage in the first place. We have now found that it is possible to provide a friction modifier which is particularly effective in countering the occurrence of slippage arising on gradients and the like while still producing lubrication effective to reduce noise and wear arising from creepage in negotiating bends in the track and from lateral oscillatory movement.